IlO
the slice preparation (Zhang and Wu) [115], and sharp electrodes in the slice preparation
(Attwell and Wilson) [5]. Compared with Zhang and Wu, our whole-mount estimate of Ra
is less than half the Ra measured in the slice preparation (0.89 GΩ vs 2 GΩ). In making the
slice preparation, one half of the surrounding network is sheared away. Because our results
show that only a small part of the current flow between the two rods is through indirect
pathways, we would not expect to see a very large difference. However, the 2 GΩ estimate
is still larger than our estimate of Rc, which suggests that in making the slice preparation,
some of the gap junctional connections between the adjacent cells could have been sheared
away.
In comparison to Attwell and Wilsons estimate of 0.3 GΩ [5], our estimate is notably
larger. They report a lower network input impedance Rin of 90 MΩ compared to our 137
MΩ estimate, which could be due to cell damage from the sharp electrodes they used. This
underestimate of Rin could have led them in turn to underestimate the coupling resistance
Rc, because it is a determinant of the input impedance (equation 5.15).
While our experiments represent the most rigorous estimates to date of the apparent
coupling resistance Ra in the salamander retina, they are also not without drawbacks. In
order to accurately estimate Rc from Ra, we had to calculate the input impedance the each
cell using recordings we made of the pair. Due to inhomogeneities in the network, the
impedances of the driver and follower cell could be different. However, this difference would
be expected to be averaged out over the many pairs we recorded. Second, the estimate of the
length constant λ2o from the data relies on the assumption that the follower cell was patched